CCDC42

CCDC42 is a coiled-coil structural protein essential for sperm flagellum biogenesis and head-tail coupling apparatus (HTCA) integrity during spermiogenesis (PMID:26945718). Its expression is restricted to brain and developing testes, consistent with a dedicated role in spermatid differentiation (PMID:26945718). In spermatids CCDC42 localizes to the manchette, the connecting piece (HTCA), and the sperm tail, and it also marks the centrosome in somatic cells (PMID:31475146). Mechanistically, CCDC42 operates within a sperm-specific cytoskeletal protein network: it physically interacts with the outer dense fiber proteins ODF1 and ODF2 (PMID:31475146), with the manchette/tail protein CCDC38 (PMID:35587122), and with CCDC146 (PMID:38038747); loss of Ccdc146 does not alter CCDC42 protein levels, placing CCDC42 upstream of or independent from CCDC146 in this pathway (PMID:38038747). CFAP58 also binds CCDC42 and acts to stabilize it during intra-manchette/intraflagellar transport (PMID:38602507), integrating CCDC42 into the transport machinery that builds the sperm flagellum and connecting piece.

1. 2016 High

   Established whether CCDC42 has a non-redundant function in male reproduction by testing its requirement for sperm formation in vivo.

   Evidence Loss-of-function knockout mouse with histological and phenotypic sperm analysis

   PMID:26945718

   Open questions at the time

   • Did not define the molecular activity of CCDC42 at the HTCA
   • Did not identify interacting partners
   • Mechanism linking CCDC42 loss to absent flagella unresolved
2. 2016 Medium

   Mapped where CCDC42 is expressed to confirm tissue specificity consistent with its spermiogenesis phenotype.

   Evidence Expression analysis in loss-of-function mouse model

   PMID:26945718

   Open questions at the time

   • Brain expression not connected to any function
   • Subcellular localization not addressed at this stage
3. 2019 Medium

   Determined the subcellular sites at which CCDC42 acts, linking it to the manchette, HTCA, sperm tail, and the somatic centrosome.

   Evidence Immunofluorescence localization in mouse spermatids and somatic cells

   PMID:31475146

   Open questions at the time

   • Single-lab localization without orthogonal validation
   • Functional significance of centrosomal localization in somatic cells not tested
4. 2019 Medium

   Identified the first direct binding partners, placing CCDC42 in the outer dense fiber/cytoskeletal protein network.

   Evidence Co-immunoprecipitation/pulldown in spermatids

   PMID:31475146

   Open questions at the time

   • Interaction interface and stoichiometry undefined
   • Functional consequence of ODF1/ODF2 binding not tested by mutation
5. 2022 Medium

   Extended the network to CCDC38 and showed cooperative function in flagellum biogenesis.

   Evidence Reciprocal Co-IP in mouse testis plus CCDC38 knockout

   PMID:35587122

   Open questions at the time

   • Direct versus indirect nature of the interaction not resolved
   • Order of assembly within the complex unknown
6. 2023 Medium

   Resolved epistatic placement by showing CCDC42 protein is unaffected by Ccdc146 loss, positioning CCDC42 upstream of or independent from CCDC146.

   Evidence Co-IP, interaction modeling, and Ccdc146 knockout with western blot

   PMID:38038747

   Open questions at the time

   • Reciprocal test (effect of Ccdc42 loss on CCDC146) not reported here
   • Mechanism of co-functioning not defined
7. 2024 Medium

   Implicated CCDC42 in intra-manchette/intraflagellar transport by identifying CFAP58 as a binding and stabilizing partner.

   Evidence Co-IP and Cfap58 knockout with assessment of CCDC42 localization/stability

   PMID:38602507

   Open questions at the time

   • Cargo-stabilization role inferred from KO, not directly reconstituted
   • Transport mechanism and motor coupling unresolved

• The biochemical activity of CCDC42 and the architecture/assembly order of its manchette-HTCA complex remain undefined.
• No structural model of CCDC42 or its complex
• No defined enzymatic or transport activity
• Human disease relevance not established in the corpus